1. Field of the invention
The present invention relates to a polymerization catalyst system, and more particularly, to an olefin polymerization catalyst system containing a Ziegler-Natta polymerizing catalyst and an alkoxysilane as an electron donor.
2. Description of the Prior Art
The polymerization of olefin always involves the use of a catalyst to provide polymers having desirable properties. The voluminous number of catalysts, denoted as Ziegler-Natta-type catalyst systems, are known in the art. In general, an olefin polymerization catalyst system includes an organometal compound of the Group I.about.III and a transition metal halide. Organic compounds of Al, Zn, Be, Mg, etc., for example (CH.sub.3).sub.3 Al, (CH.sub.2 CH.sub.3).sub.3 Al, (CH.sub.2 CH.sub.3).sub.2 ClAl and (CH.sub.2 CH.sub.3)Cl.sub.2 Al are used as the organometal compound. Halides of Ti, V, Zr, Cr, etc., for example TiCl.sub.4, TiCl.sub.3, TiBr.sub.3, VCl.sub.4 and VOCl.sub.3, are used as the transition metal halide.
In olefin polymerization, reaction conditions and catalysts can be selected according to the kind of olefin, polymerization type and other factors.
Polymerization catalyst activity is measured by weight of polymer per unit weight of catalyst in 2 hours (Kg-polymer/g-catalyst.2hr). In addition to catalyst activity, for polyolefin production in commercial scale, large isotacticity and large bulk density (g/ml) are desirable.
In order to increase the yield of polyolefin, improvement of a catalyst as well as improvement of activation of a catalyst and development of a cocatalyst or an electron donor have been studied.
An electron donor functions to decrease the production of atactic polymer and control or increase the production of isotactic polymer in olefin polymerization. Although a broad range of compounds are known to be used as an electron donor, certain compounds are especially compatible to a particular compound or a group of compounds. Therefore, properties such as the isotactic index of products, molecular weight distribution, melt flow etc. can be improved by selection of a high performance electron donor.
Parodi et al. reports that alkoxysilane is useful as an external electron donor in an olefin polymerization to increase isotactic products (Eur. Pat. 45977). Seppala et al. reports that the effect of alkoxysilane as an external electron donor depends on the number and the size of its alkoxy group and the size of hydrocarbon substituted in silicon atom, and too much sterically-hindered hydrocarbon does not make effects although the isotactic product increases in proportion to the size of the hydrocarbon.
U.S. Pat. No. 4,816,433 sets forth cyclohexyl or cyclopentyl alkoxysilane is useful in producing a polyolefin having high stereoregularity.
U.S. Pat. No. 4,927,797 sets forth an olefin polymerization catalyst system containing an electron donor compound of the formula: ##STR2## wherein R.sub.1 is a bulky, basic alkyl or cycloalkyl group containing at least one secondary or tertiary carbon atom; R.sub.2 and R.sub.3 are alkyl or aryl groups; and R.sub.4 is an alkyl group with a primary carbon atom attached to the silicon atom.
U.S. Pat. No. 5,296,624 sets forth that sterically-hindered organosilanes such as neopentyl alkoxysilanes or trimethyl alkoxysilanes are effective electron donors in polymerization reactions.